For the past several years, considerable effort has been expended on the development of a miniature fiber optic pH probe for physiological use. With the first generation of probes and support equipment the feasibility of optically measuring pH via a pH sensitive dye was demonstrated and many subtleties associated with the probe construction and use made apparent. With this knowledge an improved probe and multichannel support system was designed and constructed and is presently being used to measure pH in the wall of beating canine hearts. Within the last year further refinements in the probe design construction have reduced its response time to step change in pH while signal and proccessing modifications have extended the range of the probe from .4 pH units (7.0 to 7.4) to 2.0 pH units (6.0 to 8.0). In addition, preliminary results indicate that LED's and photodiodes may be used with the new probe design.